1. Field of the Invention
This invention generally relates to laser scanning systems for reading indicia having portions of different light reflectivity such as bar code symbols and, more particularly, to a lightweight, multi-component, portable laser diode scanning head supportable by a user and aimable at each symbol to be read. Still more particularly, this invention relates to deactuating at least one of the components, e.g. the laser diode, during part of each scan of the symbol; minimizing the dead zone within the field of view of a detector in the head, in which dead zone specular reflection may prevent a successful decode and reading of the symbol; and resisting separation of parts of the head unless an electrical cable connecting the head to the remainder of the system is first removed.
2. Description of Related Art
Various optical readers and optical scanning systems have been developed heretofore to optically read bar code symbols printed on labels affixed to objects in order to identify the object by optically reading the symbol thereon. The bar code symbol itself is a coded pattern comprised of a series of bars of various widths, and spaced apart from one another to bound spaces of various widths, said bars and spaces having different light-reflecting characteristics. Such readers and systems electro-optically decoded the coded patterns to a multiple alpha-numerical digit representation descriptive of the object. Scanning systems of this general type, and components for use in such systems, have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,593,186; 4,496,831; 4,409,470; 4,460,120; 4,607,156; 4,673,805; 4,736,095; 4,758,717 and 4,760,248, as well as in U.S. Ser. Nos. 196,021; 7,775; 944,848; 138,563; 148,438; 148,669; 148,555 now U.S. Pat. Nos. 4816660, 4808804, 4816661, 4871904, 4806742, 4825057 and 4845350 respectively; and 193,265 now U.S. Pat. No. 5124540; all of which have been assigned to the same assignee as the instant application and are incorporated herein to show the state of the art.
As disclosed in some of the above patents and applications, a particularly advantageous embodiment of such a scanning system resided, inter alia, in optically modifying and directing a laser light beam from a hand-held head which was supported by a user; aiming the head and, in some cases, the laser beam itself at a symbol to be read; repetitively scanning the laser beam and/or the field of view of a detector across the symbol; detecting the laser light reflected off the symbol during scanning; and decoding the detected reflected light.
The laser beam is optically modified and focused to form a beam spot having a minimum beam cross-section or waist at a reference plane. A symbol can be read at either side of the reference plane. For ease of description, a symbol located between the reference plane and the head is defined as a "close-in" symbol, whereas a symbol that is located on the other side of the reference plane away from the head is defined as a "far-out" symbol. The term "close-in" symbol is also intended to cover the situation where the symbol actually is contacted by the head, or where the reference plane is located immediately outside the head. The range between minimum and maximum distances at which the system can read a symbol is often defined as the "depth of field". The depth of field is, of course, different for symbols of different densities.
A problem associated with known laser scanning systems involves the so-called dead zone or area within the field of view of the scanner in which specular reflection may prevent a successful decoding and reading of the symbol. When a light beam impinges on a surface, such as a symbol on a label, the reflected light has a specular component and a scattered component. The scattered component radiates in all directions, and its intensity is proportional to the contrast between the darker bars and the lighter spaces of the symbol. It is the scattered component of the reflected light which is detected and processed by the system to decode and read the symbol.
The specular component, on the other hand, is a mirror-like reflection wherein the light is reflected according to the principle that the angle of reflection is equal to the angle of incidence. The specular component is the major constituent of the reflected light, but is not that constituent which is used by the system to decode and read the symbol, because the intensity of the specular component is more dependent on surface finish as opposed to the contrast between the darker bars and the lighter spaces of the symbol. The specular component, also called glare, is typically too bright, particularly when a symbol is printed on a label having a glossy finish or overlaid with cellophane or film packaging, and can overload and "blind" the detector.
When the system includes a hand-held head having a handle which is held by a user, and is pointed at a symbol, the untrained user expects the head to work best when the laser beam exiting an exit window on the head is aimed at a right angle to the plane in which the symbol lies. The user tends to hold the handle approximately parallel to that plane. However, this is the very position where the dead zone is centered and has its maximum effect, because specular light striking the symbol at an incidence angle of 90.degree. will return at a reflection angle of 90.degree., thereby blinding the detector.
It would be desirable to minimize the dead zone and enable even the untrained operator to manipulate the hand-held head in an expected manner without causing the system to fail to read due to specular reflection.
Yet another problem associated with known laser scanning systems, particularly the field-portable type powered by an on-board battery, resides in minimizing power consumption and premature draining of the battery. In systems of the type exemplified by U.S. Pat. No. 4,251,798, the gas laser was left on all the time, not only during scanning, but also between scans. In later systems of the type exemplified by U.S. Pat. Nos. 4,387,297 and 4,409,470, the laser was shut off between scans to save power.
Although the later systems did consume less power, the power drain was still undesirably high, particularly when visible laser diodes are used. The diode is the largest consumer of electrical current of all the components in the head. Operating the diode, except when absolutely necessary to scan the symbol, decreases not only the battery life, but also decreases the working lifetime of the diode itself.
The diode is also the greatest source of waste heat in the head, and the working lifetime of the diode also is decreased by being exposed to its own heat. By operating the diode only when necessary, the diode will generate less heat, thereby enabling the head to run cooler than heretofore, enabling the head to be used in hotter ambient environments than heretofore, and enabling the diode to last longer in the field.
Yet an additional drawback involves safety, wherein it is desired to prevent one from separating different housing parts of the head without first being certain that all power to the head is first disconnected.